1. Field of the Invention
The present invention relates to security cameras, and, more particularly, to security cameras that are concealed within a hemispherical dome.
2. Description of the Related Art
Surveillance camera systems are commonly used by retail stores, banks, casinos and other organizations to monitor activities within a given area. The cameras are often provided with the capability to pan and tilt in order to acquire images over a wide domain. The tilt of the camera generally refers to the pivoting of the camera about a horizontal axis that is parallel to the floor, such that the lens of the camera may tilt between an upwardly pointing position and a downwardly pointing position. The pan of the camera refers to the rotation of the camera about a vertical axis that is perpendicular to the floor, such that the lens may scan from side to side. The cameras may also be able to zoom in order to reduce or enlarge the field of view. Oftentimes, each camera is linked to video display units in a security surveillance room with surveillance personnel monitoring the multiple video display units.
Surveillance cameras may be mounted within a hemispherical dome window constructed of a material that is transparent when viewing outward and only partially transparent when viewing inward to inhibit unauthorized individuals from determining the area being viewed by the camera. Similarly to sunglasses, the window may be tinted or provided with a thin metalized layer.
The prior art pendant housing shown in FIGS. 1-3 includes a clear window that is stationary relative to the camera pan/tilt/zoom movement inside. The clear window is hemispherical in shape and is mechanically engaged with the aluminum housing in a manner that compresses a static rubber seal to prevent ingress of water and dust. The objective diameter of the camera lens requires the lens optical axis to be offset from the geometric center of the hemispherical window in order for the camera to view entirely through the hemisphere at a horizon (i.e., horizontal or 0 degree tilt angle).
Advantages of static hemispherical windows include: a) a cost-effective means of meeting basic requirements of environmental protection, and b) the covertness afforded by the outer surface of the window not moving when the camera moves. However, when high-zoom lenses and/or high definition imagers are used, a static hemispherical window has significant optical limitations: first, refractive distortions are introduced by the offset of the lens optical axis from the geometric center of the hemispherical window; second, variations in the local optical properties of the hemispherical window are more noticeable as the lens moves (panning and tilting) relative to the window; and third, convection of air and moisture inside the window can introduce optical variations. In addition, the production of a hemispherical shape using the cost-effective plastic injection-molding process further limits optical performance due to: first, geometric limitations associated with injection molding a hemispherical shape; second, optical properties of plastics such as PMMA (acrylic) or PC (polycarbonate) vs. glass; third, molded-in stress that creates localized optical distortions; and fourth, limited application of coatings to enhance optical performance.
The window may have a compound shape including a hemisphere and a cylindrical section per U.S. Pat. No. 7,306,383 and Japanese Patent Publication 2006-033704. However, if the lens optical centerline was shifted to the geometric center of the hemisphere, there would be distortion visible when viewing the horizon due to the transition from hemispherical to cylindrical, and additional refractive distortion in the cylindrical section.
Some of the limitations of a stationary hemispherical window can be overcome by attaching the window to the same moving pan stage that supports the camera lens tilt pivot, as shown in FIGS. 4 and 5. The lens still tilts relative to the window, however, so the optical viewing area of the window is a curved strip of the hemisphere, as shown in FIG. 6.
Problems associated with the design of FIGS. 4-6 include: a) variations in local optical properties are visible as the lens tilts relative to the window; and b) convection of air and moisture inside the dome can introduce optical variations. Additionally, the production of a curved hemispherical strip shape using the cost-effective injection molding process further limits optical performance due to: first, the poorer optical properties of plastics such as PMMA (acrylic) or PC (polycarbonate) as compared to glass; second, the molded-in stress that creates localized optical distortions; and third, the limited application of coatings to enhance optical performance.
The region shown in FIG. 6 does not need to be a separate part. As shown FIGS. 4-5, the “eyeball” could be split on an angle to provide horizon-to-vertical tilt range viewing through a simple, and therefore easily manufactured, hemisphere. The pan bearing may be placed at the top where the “small diameter moving pan seal” is noted. The benefits of a small diameter seal include cost and lower friction for the pan drive to overcome. The slip ring may also be located in this area.
What is neither disclosed nor suggested by the prior art is a dome camera system that overcomes the above-described problems with the prior art.